Add case and caseOn as instance methods for types

[pelotom]

Add instance methods for case and caseOn so you can do e.g.

const { Just, Nothing } = Type({
  Just: [T],
  Nothing: []
})

const m = Just(3)

m.case({
  Nothing: () => 0,
  Just: (n) => n + 2
})

[paldepind]

Thank you. It seems sensible.